1. Field of the Invention
The present invention relates to a process for preparing a highly expanded polyvinyl chloride foam product.
Description of the Prior Art
In spite of its excellent physical properties, such as fire retardance and weather resistance, expanded polyvinyl chloride foam (hereinafter called PVC foam) is not used with the wide-ranging application of, for example, polyurethane foam or polyolefin foam. The reasons for this include the fact that a highly expanded PVC foam (low density foam) is technically difficult to produce while, on the other hand, a low expanded PVC foam is not only expensive to manufacture but also is far inferior to polyurethane foam and polyolefin foam in performance for such applications as cushioning and heat insulation.
In the conventional method for preparing a highly expanded PVC foam, high-pressure equipment is required because of the need to foam the PVC with an inert gas under elevated pressure, or by decomposition of a chemical blowing agent. The productivity rate of said methods is quite low, because these methods require batch type systems. Consequently, the foam obtained in these manners is very expensive and is used only where specifically required. Thus, pressurized expansion methods are not universally employed industrially, even though a highly expanded PVC foam can be obtained thereby.
Furthermore, highly expanded PVC foam cannot be made using such processing equipment as calenders or extruders which are usually used in the manufacture of PVC foam. The expanded PVC foam obtained by conventional calender processing is prepared by blending a plasticizer, a stabilizer, and a chemical blowing agent with an ordinary PVC. Calendering must occur at a temperature such that the chemical blowing agent is not decomposed. After calendering, the product is heated in hot air to effect expansion. Unfortunately, expansion by calender processing is only applicable to the manufacture of sponge leather, etc., since the degree of expansion obtainable is at most only 3 to 5 times. If the extent of foaming is increased beyond this, the cells tend to break down, which results in the formation of a foam which is unsuitable for practical use.
Another conventional method for manufacturing an expanded PVC foam is by use of a chemical blowing agent in combination with a stabilizer and other additives. After the blend with PVC is formed, the blend is milled in an extruder to cause the chemical blowing agent to decompose. The gas produced is pressurized and dispersed in the cylinder containing the melt and is expanded at the same time that the blend is extruded through a die. Even using this method, however, the extent of foaming is limited to 3 or 4 times, and it is very difficult to obtain an expanded foamed product which is suitable for many commercial applications.
Two other techniques attempted in the prior art for preparing PVC foam include the use of a plastisol. One of those techniques is the elastomer process which uses carbon dioxide, and the other is the "mechanical froth process" which requires the addition of specific surface active agents followed by mechanical whipping. In the elastomer process, a special apparatus is needed to effect the absorption of the carbon dioxide by the plastisol under elevated pressure at low temperatures. This manufacturing technique is not only complicated, but also the achievable extent of foaming is limited to, at most, 20 times. In the mechanical froth process, the extent of foaming is limited to about 5 to 10 times, and a product having a density comparable to polyurethane foam of 0.02 g/cc cannot be obtained.
It would be highly desirable to have a process for efficient and effective preparation of highly expanded polyvinyl chloride foam on an industrial scale.